The invention relates to the design and manufacture of single-vision ophthalmic spectacle lenses.
Vision correction for myopia (nearsightedness) and hypermetropia (farsightedness) can be accomplished using spectacle lenses having appropriate spherical curves on the anterior (outer) and posterior (inner) surfaces. Astigmatism, with or without either of these errors, can be corrected if one of the surfaces is toroidal, or spherocylindrical, having different refractive powers or magnifications along two principal axes or meridians typically separated by 90 degrees. The toroidal surface can be placed on the posterior side of the lens. See, for example, M. Dowaliby, "Practical Aspects of Ophthalmic Optics," 3rd Edition, Chapter 6, New York: Professional Press Books, 1988. Corrective spectacle lenses utilize optical designs having appropriate curves, most typically in lens series, for which the anterior and posterior curves are set out in a manner that can permit efficacious manufacture without sacrificing optical correction.
A number of technologies exist for the manufacture of prescription lenses using corrective lens curves which can be related to the optical material being fabricated. Examples of optical materials include: (a) optical glass, for which the principal method of manufacture includes grinding and polishing of the lens surfaces; (b) thermosetting plastic monomers, such as allyl diglycol carbonate bis (CR-39.RTM.), for which the principal method of manufacture includes casting and curing the liquid monomer in a casting cell having a pair of optical quality casting molds with an apparatus for properly spacing the molds and retaining the monomer; and (c) thermoplastic materials such as polycarbonate (e.g., LEXAN.RTM.) or polymethyl methacrylate (PLEXIGLAS.RTM.), for which the principal method of manufacture includes injection molding of the thermoplastic resin, using heat and pressure to form the optical object in a molding cavity containing suitable optical quality molding surfaces.
The specific choice of anterior and posterior curves can affect the degree of satisfactory vision correction as perceived by the wearer. For example, M. Jalie, "The Principles of Ophthalmic Lenses," 4th Edition, Chapter 18, London: The Association of British Dispensing Opticians, 1988, describes in detail the effects of anterior and posterior curve selection on satisfactory vision correction. This approach to optical correction can result in a large number of unique anterior and posterior curves.
Ophthalmic prescription lenses can be manufactured using a posterior base curve series. In a posterior base curve series, the same posterior curve is used to prepare lenses over prescriptive ranges of 1 to 2 diopters while altering the anterior curves appropriately, typically in 0.25 diopter increments, to obtain the correct refractive power. The posterior base curve series method is a typical method of manufacturing prescription lenses in a one-step process where both anterior and posterior surfaces are provided as integral parts of the manufacturing process by the lens manufacturer. The completed article provided by the lens manufacturer to an optician is generally referred to as a finished uncut, indicating that the lens requires only proper rotational orientation for correction of astigmatism, if any, and machining of the edge to fit the patient's spectacle frame.
A degree of latitude exists regarding the value of the anterior curve for a specific prescriptive correction. The anterior curve of a prescriptive correction providing acceptable optics can vary by as much as .+-.1 diopter about a nominal value provided it has an appropriate posterior curve and central thickness to preserve correct refractive power. See, for example, I. M. Borish, "Clinical Refraction," 3rd Edition, Chapter 26, New York: The Professional Press, Inc., 1970, and J. K. Davis et al., "The Design of a General Purpose Single Vision Lens Series," American Journal of Optometry & Archives of American Academy of Optometry, 42(4):203-236 (1965).
The anterior surfaces for all prescriptions are spherical and the posterior surfaces can be either spherical or toroidal, the latter being used for astigmatism correction. See, for example, Davis, et al. U.S. Pat. No. 3,434,781. In one example, a finished single-vision lens series for spherical correction of myopia or hypermetropia, covering the range of +4.00 diopters to -8.00 diopters in 0.25 diopter steps, can utilize 49 unique anterior surface spherical curves combined properly with 6 unique posterior spherical curves when manufactured to acceptable limits of refractive power accuracy.
Spherical single-vision ophthalmic lens series of the anterior base curve type can follow the guidelines of anterior curve selection, as set out by, for example, Davis, Journal of Optometry & Archives of American Academy of Optometry, 42(4):203-236 (1965), and Borish, "Clinical Refraction," 3rd Edition, New York: The Professional Press, Inc., 1970, with different manufacturers selecting differing portions of each nominal anterior sphere power range. Typical selections of anterior curves tend to prefer the lower valued or flat portion of the allowable base curve range, to improve cosmetic appearance (e.g., cosmesis).
Under some circumstances, additional posterior toroidal surfaces can be required to correct for astigmatism as well as myopia or hypermetropia. For example, a full range of prescriptive lenses, correcting for 3 diopters of astigmatism, in addition to the aforementioned 13 diopters of sphere power for myopia or hypermetropia correction, incorporates 637 prescriptions or stock keeping units (SKUs). For each required posterior spherical surface, an additional 12 toroidal surfaces are required for the astigmatism correction range indicated.